It is a known problem with three-phase machines such as synchronous generators, asynchronous generators and the like that shaft voltages or bearing currents are formed, wherein such shaft currents or shaft voltages can have different causes such as electrostatic charges of the generator shaft, external electrical fields which capacitively couple shaft voltages or shaft currents into the generator shaft or magnetic asymmetries of the shaft environment. The formation of uncontrolled current circuits can in this respect result in damage to components included in this current circuit due to current effect and spark erosion.
Different measures have already been proposed to counter this problem. For example, with insulated bearings, in particular with large machines and high frequencies, the desired effect cannot always be achieved due to the relatively thin insulation layer on the ring of the bearing whose thickness typically lies in the micrometer range since capacitive currents can bridge the bearing insulation due to the capacitor effect.
Insulated bearing shields or insulated bearing seats are frequently used in synchronous machines, wherein a galvanic separation of the outer bearing ring takes place with respect to the housing by a specific arrangement of insulated parts such as screws, sleeves and the like, with the shaft here typically being sealed at both sides by a slide seal ring.
Hybrid bearings in which the races are produced from steel and the roller bodies are produced from ceramics can represent a remedy. The spacing between the races is in particular relatively large on a use of balls as roller bodies so that a good insulation can be achieved, with in this case the shaft typically being able to be sealed by labyrinth seals.
Further measures for reducing or suppressing wave currents are known from documents DE 35 11 755 A1 or DE 197 42 622 A1.
With contaminated environmental conditions, the previously used measures are, however, sometimes not sufficient. Current-conducting bridges can in particular occur in the region of the shaft seals in connection with electrically conductive dust or also under unfavorable production tolerances.
It is the underlying object of the present invention to provide an improved electric machine and an improved labyrinth seal of the initially named kind which avoid the disadvantages of the prior art and further develop the latter in an advantageous manner. Wave currents should in particular also be securely suppressed in contaminated environmental conditions in the region of the shaft seal at three-phase machines, in particular synchronous generators, whose shaft is galvanically separated with respect to the housing by insulated bearings, without creating complicated structures for this purpose.
In accordance with the invention, the named object is achieved by an electric machine having a shaft rotatably supported in a housing and having a labyrinth seal with labyrinth contours which engage into one another for sealing the housing with respect to the shaft, wherein at least one side of the labyrinth seal is at least provided with an insulating coating in a region of the labyrinth contour, and by a labyrinth seal for sealing a housing of a three-phase machine with respect to a shaft rotatable with respect to the housing having at least two seal parts which have labyrinth contours which engage into one another, wherein at least one side of the labyrinth seal is at least provided with an insulating layer in a region of the labyrinth contours.
It is therefore proposed to make electrically conductive bridges, which can in particular occur in the region of the labyrinth contours, harmless by an insulation of the labyrinth seal such that such electrically conductive bridges are admittedly still formed, for example by conductive dust, but can no longer transmit any current or, more precisely, can no longer be supplied with current and can no longer discharge any current. In accordance with the invention, at least one side of the labyrinth seal is at least provided with an insulating coating in the region of the labyrinth contours. If an electrically conductive bridge is formed in the region of the labyrinth contours by contamination, current can nevertheless not be transmitted from one labyrinth seal part to the other labyrinth seal part.
In an advantageous further development of the invention, both sides of the labyrinth seal can at least be provided with such an insulating coating in the region of the labyrinth contours. The named insulating coating can in this respect advantageously not only be applied in the region of the labyrinth contours, but can be provided over the total surface of at least the mutually facing sides of the labyrinth seal parts.
Such an insulating coating can be produced from a suitable plastic with which the labyrinth seal parts can be coated.
In a further development of the invention, at least one of the labyrinth seal parts, which has one of the named labyrinth contours, can in this respect also be produced completely from an insulating material, in particular from plastic, so that not only the surface of the labyrinth seal part has an insulating coating, but the labyrinth seal part is rather also formed as insulating in cross-section or the solid material is formed as insulating. The desired insulating effect can be maintained over a longer period by such a formation of the labyrinth seal part completely comprising insulating material even though wear occurs at the surface of the labyrinth seal, for example by contamination.
In a further development of the invention, both labyrinth seal halves can each be produced from an insulating material, in particular plastic.
The shaft to be sealed with respect to the housing by the named labyrinth seal can be rotatably supported at the named housing by at least one hybrid rolling bearing whose roller bodies comprise a different material than its races. The races can in particular be produced from steel and the roller bodies, for example balls, from ceramics. The propagation of shaft currents can also be suppressed in the region of the shaft support by the use of such a hybrid rolling bearing.
In an advantageous further development of the invention, a respective labyrinth seal having an insulating coating and/or composed of insulating material can be provided at both sides of the named hybrid bearing.
The named labyrinth seal can be used in an advantageous manner with different three-phase machines whose shaft is galvanically separated with respect to the housing by insulated bearings. The use of the named labyrinth seal can in particular be of advantage with synchronous generators.
The invention will be explained in more detail in the following with reference to a preferred embodiment and to associated FIGS.